The present invention relates to carbon nanotubes, and methods for their synthesis. In particular, the present invention provides catalysts and conditions for low temperature synthesis.
Carbon nanotubes (“CNTs”) are helical microtubules of graphitic carbon. CNTs are of particular technical and commercial attention in part because of interesting and sometimes unique properties that make them attractive in many potential applications. For example, their high strength-to-weight ratio makes CNTs some of the strongest materials ever made. Whereas traditional carbon fibers have a strength-to-weight ratio about forty times that of steel, CNTs have a strength-to-weight ratio of at least two orders of magnitude greater than steel. CNTs and other tubular nanostructures with tunable dimensions below 100 nm are of increasing interest for applications such as molecular electronic devices, field-emission display panels, hydrogen storage, and micro-electromechanical systems (MEMS). There is significant worldwide research related to improving CNT synthetic methodology to improve yields and lower overall cost.
The simplest carbon nanotubes (“SWNTs”) are single-walled, formed from a graphitic sheet rolled up on itself with a helical pitch and joined seamlessly at the edges. Frequently, such tubes are a closed with a conical cap, having diameters of from 10 to 20 Angstroms. Multi-walled carbon nanotubes (“MWNTs”) are consist of a plurality of concentric tubes, each formed by closure of a graphitic sheet, with the distance between concentric tubes being about 0.34 nm. MWNTs may contain only two concentric tubes, or as many as fifty or more concentric tubes.
The preparation of CNTs is possible using diverse methods, such as those described in C. Journet and P. Bernier, Appl. Phys. A 67, 1-9 (1998). One method is through electric arc discharge generated between two closely spaced graphite electrodes in an inert atmosphere, such as helium or argon. A plasma, with a temperature on the order of 4000 K, is created between the electrodes, subliming carbon from the anode onto the cathode. Normally only MWNTs are formed by this method, but if a metal is introduced as a catalyst (e.g., Co, Ni, Fe, Lu, and combinations thereof) SWNTs may be formed. In another method, laser ablation is used to vaporize graphite in an inert atmosphere. In such methods, a laser beam is scanned across a heated graphite target area over which flows an inert gas such as helium or argon. Carbon species produced are swept by the gas onto a cooled target, e.g., copper.
High-temperature gas phase decomposition of hydrocarbons also can be utilized to form CNTs. For example, nitrogen containing 10% acetylene passed over a metal at 500-1100° C., resulting in the formation of multi-walled carbon nanotubes. Chemical vapor deposition (CVD) is the method of choice, partially due to its relatively inexpensive apparatus and facile industrial scale-up. Although CVD offers the additional benefit of significantly lower deposition temperatures than arc-based techniques, a temperature range of 700-1000° C. is still necessary, which precludes the use of temperature-sensitive substrates that may not withstand such deposition temperatures. Recently, growth of MWNT nanotubes and nanorods has been reported at 200° C. from benzene-thermal-reduction-catalysis (BTRC), which uses chlorine-containing precursors with in situ alkali metal facilitated reduction.